I'm going to guess it's the latter. I'm unable to log into some of my existing gmail pseudonymous aliases because google refuses to let me log in without providing a phone number (ostensibly for my own security, IIRC to enable 2fa, even though it doesn't make any sense).
An existing email account is something worth protecting. It's OK to allow a few extra false negatives while signing up, compare to accessing an existing account.
Wouldn't it be up to me to decide if I want to enable 2fa?
Provided Google has a lot of info about me, they can probably tie those accounts to me anyway, but I'd rather not formally associate them with my public identity.
